This invention relates to the continuous casting of molten metal, especially steel, and more particularly to the shrouding of molten metal pouring streams between a teeming vessel and a continuous casting mold, to protect such stream from atmospheric reoxidation.
In the continuous casting of molten metal such as steel, hot metal is teemed from a ladle into an intermediate pouring vessel, called a tundish, which is positioned above a continuous casting mold. The tundish has a pouring nozzle in its bottom wall. A tundish for continuous casting has a pouring nozzle for each strand to be poured. For instance, when continuously casting billets into a six strand continuous caster, the tundish would require six pouring nozzles.
Atmospheric oxygen contacts the pouring stream causing reoxidation of the stream as it flows between a tundish and a mold, resulting in the carryover and entrapment of undesirable oxide inclusions in each continuously cast strand. A continuous casting having inclusions trapped therein may be unacceptable for many applications. Certain quality steel grades require a high degree of product cleanliness. To alleviate the problem of reoxidation of a molten metal pouring stream, several methods have been developed to surround the pouring stream with a protective atmosphere, usually within a tubular enclosure. Holmes U.S. Pat. No. 3,439,735; Lyman U.S. Pat. No. 3,572,422; Newhall et al U.S. Pat. No. 3,616,843; and Pollard U.S. Pat. No. 3,908,734 teach shrouding of molten metal pouring streams with an inert or reducing gas. Some shroud enclosures are manufactured from refractory materials and are termed "refractory pouring tubes" since they project downwardly from the bottom of a tundish to a location beneath the surface of the metal in the mold as shown by Mills et al U.S. Pat. No. 3,517,726. This type of pouring tube affords no access to the tundish nozzle.
It is preferable to have a shroud enclosure form a gas-tight seal with the pouring vessel and extend downwardly as far as possible toward or into the mold. It is also desirable to have the tubular enclosure or shroud readily removable from the active position surrounding the pouring stream when the pouring nozzle becomes plugged and access to the nozzle is required. At such times an oxygen lance must usually be inserted in the pouring nozzle to re-establish the pouring stream. When a breakout occurs in the skin of the casting after it exits from the casting mold, it is desirable to shut off the flow of molten metal to the mold at which time a launder is positioned beneath the pouring nozzle to divert the stream from the mold. At other times, it is desirable to terminate casting by the insertion of a chill plug into the tundish nozzle to shut off the flow of metal.
It has been determined experimentally that the distance from the top of the liquid level in the mold to the bottom of the shroud enclosure is critical in limiting the percentage of oxygen in the gaseous environment within the shroud itself. Reoxidation of this molten metal stream will not occur if the oxygen concentration in the shroud is less than 0.80%, but will occur if the oxygen concentration is above 0.80%, forming undesirable oxide inclusions which will show up as defects in the cast product.